Faraday rotator suitable for use in an optical isolator and laser diode module incorporating the same

Abstract: Disclosed is an optical isolator that permits a low-profile construction. Two rod-like permanent magnets (36) are disposed on both sides of a faraday element (18) in parallel to an optical axis (16). The two permanent magnets are sandwiched and fixed between two ferromagnetic frames (38) each having a light path hole (40) therein. The ferromagnetic frames are each provided with a protrusion (46) for guiding the magnetic flux generated by the permanent magnets to the faraday element.

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a Faraday rotator suitable for use in an optical isolator, and a laser diode module incorporating the same an optical isolator. 2. Description of the Related Art An optical isolator may be used in combination with a semiconductor laser diode (LD), to prevent back-reflections, caused at optical fiber connections, from affecting the stable operation of the LD when transmitting light, emitted from the LD, through an optical fiber. In many cases, the optical isolator for such use is assembled in an LD package containing an LD. In this type of optical isolator, a Faraday element is positioned in the center of a hollow cylindrical permanent magnet so a magnetic field in the direction of the optical axis is applied to the Faraday element placed on the optical axis. In other words, the Faraday element placed on the optical axis is surrounded by a cylindrical permanent magnet. When an optical isolator is assembled in an LD package for use, the optical isolator is mounted onto a frame of the LD package by welding or other means. In this case, it is desirable that the optical isolator be made as thin as possible to enable the size of the whole unit to be reduced. However, with prior known optical isolators, it has been difficult to reduce the thickness and achieve a low-profile construction because, as described above, a permanent magnet in the shape of a hollow cylinder is used to enclose the Faraday element. A. Shibukama et al. 'Compact optical isolator for near-infrared radiation', Electronics Letters, Vol. 13, Nr. 24, 1977, pages 721-722, shows a Faraday rotator composed of a permanent magnet cylindrical and two yokes, each in shape of a cylinder with a hole for light beam transmission. Between the two yokes both the permanent magnet and a Faraday element are sandwiched. There are also protrusion for guiding the magnetic flux from the cylindrical permanent magnet to the Faraday element. US 5 040 863 discloses a method for manufacturing an optical isolator having a Faraday rotator and a polarizer. Moreover, this document discloses an optical module incorporating such an optical isolator. Within the optical module there is disposed a laser diode. Furthermore, the Faraday element is made of a multilayer structure. SUMMARY OF THE INVENTION The present invention is intended to resolve the problem of the prior art, and an object of the invention is to provide an optical isolator capable of realizing a low-profile construction by improving the shape of the permanent magnet used in the optical isolator, and also provide a Faraday rotator suitable for use in such an optical isolator, and a laser diode module incorporating such an optical isolator. According to the present invention, there is provided an optical isolator comprising: a polarizer; a Faraday element disposed on an optical axis of the polarizer; a solid permanent magnet disposed in close proximity to the Faraday element; and a ferromagnetic member for guiding a substantial proportion of the magnetic flux generated by the permanent magnet to the Faraday element. According to the present invention, there is also provide a Faraday rotator comprising: a Faraday element; a solid permanent magnet disposed in close proximity to the Faraday element; and a ferromagnetic member for guiding a substantial proportion of the magnetic flux generated by the permanent magnet to the Faraday element. According to the present invention, there is also provided a laser diode module comprising: a polarizer; a Faraday element disposed on an optical axis of the polarizer; a solid permanent magnet disposed in close proximity to said Faraday element; a ferromagnetic member for guiding a substantial proportion of the magnetic flux generated by the permanent magnet to the Faraday element; and a laser diode disposed near said Faraday element and on the optical axis. BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 is a simplified schematic diagram showing optics in an optical isolator; Figures 2A and 2B are a perspective view and a cross-sectional view, respectively, of a prior art optical isolator; Figures 3A and 3B are a perspective view and a cross-sectional view, respectively, of an optical isolator according to one embodiment of the invention; Figure 3C is a perspective view showing a frame for the optical isolator according to the one embodiment; Figure 4 is a diagram showing the magnetic flux produced in the optical isolator shown in Figures 3A to 3C; and Figures 5A and 5B are a top plan view and a side view, respectively, of a laser diode module incorporating the optical isolator of the invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Figure 1 is a simplified schematic diagram showing optics in an optical isolator used in combination with a laser diode. In Figure 1, a Faraday rotator 12, which rotates the plane of polarization by 45°, and a polarizing prism 14, which is oriented at 45°, are arranged on the optical axis of the incident light from a laser diode 10. As is well known, the Faraday rotator 12 contains a Faraday element formed from an optically isotropic transparent medium to which a magnetic field H is applied in the direction of the optical axis. When a beam of light from the laser diode (LD) 10 is input in the forward direction to the optical isolator, that is, from the side nearer to the Faraday rotator 12, a linear-polarization component in the input beam having a prescribed plane of polarization is rotated 45° by the Faraday rotator 12, and the light whose plane of rotation is thus rotated enters the polarizing prism 14. Since the polarizing prism 14 is oriented 45° with respect to the plane of polarization of the output light of the LD 10, the light from the Faraday rotator 12 is passed through the polarizing prism 14, thus producing the output beam. On the other hand, when an input beam is fed in the opposite direction, that is, from the side nearer to the polarizing prism 14, only a beam of polarized light whose plane of polarization coincides with the orientation of the polarizing prism 14 is allowed to pass through the polarizing prism 14, then through the Faraday rotator 12, from which the beam emerges with its plane of polarization rotated 45°. In this case, the plane of polarization of the light is rotated in the same direction as the direction in which the light input in the forward direction is rotated; as a result, the plane of polarization of the light fed in the opposite direction is orthogonal to that of the input light fed in the forward direction. Therefore, when an optical isolator comprising a Faraday rotator and a polarization prism, such as shown in Figure 1, is inserted in the output light path of an LD, back-reflections from optical fiber connections, etc. do not affect the LD, and thus, unstable operation of the LD due to back-reflections can be prevented. Figures 2A and 2B show an optical isolator of the prior art. Figure 2A is a perspective view, and Figure 2B is a cross-sectional view. As shown in Figures 2A and 2B, the optical isolator comprises a Faraday element 18 and a polarizing prism 20 arranged on an optical axis 16 and housed in a hollow cylindrical permanent magnet 22, the entire construction being mounted in a frame 24 made of stainless steel or the like. The polarizing prism 20 is inserted in a recess 26 formed in the frame 24 and secured thereto by adhesion or other means. Further, the permanent magnet 22 is secured to the circumference of the disc-shaped frame 24 by adhesion or other means. The whole unit is thus fabricated as an integral construction. The Faraday element 18 is placed in a magnetic field produced by the permanent magnet 22; when a collimated beam of linearly polarized light is fed into it, the Faraday element 18 rotates the plane of polarization of the input beam by 45°. The permanent magnet 22 is magnetized in the same direction as the direction of the light path, and is formed in the shape of a hollow cylinder; with the Faraday element 18 positioned in its center, the magnetic field is applied to the Faraday element 18 in the direction of the optical axis 16. The polarizing prism 20 is a right quadrangular prism consisting of two right triangular prisms, 28 and 30, bonded back-to-back, with a polarization splitting film, formed from multiple dielectric layers, sandwiched therebetween to form a reflecting face 32. The polarizing prism 20 transmits light having a prescribed plane of polarization, and reflects and blocks light having a plane of polarization orthogonal to it. When an input beam from an LD (not shown) is fed in the forward direction into the optical isolator, that is, when a collimated beam of linearly polarized light having a prescribed plane of polarization is input from the side nearer to the Faraday element 18 toward the polarizing prism 20, the plane of polarization of the light is rotated 45° by a Faraday rotator formed from the Faraday element 18 and permanent magnet 22, and the thus rotated light enters the polarizing prism 20. Since the polarizing prism 20 is oriented 45° with respect to the plane of polarization of the input beam, the light from the Faraday rotator is allowed to pass through, and output light emerges from a light path hole 34 formed in the frame 26. On the other hand, when an input beam is fed in the opposite direction to the optical isolator, that is, when light is input from the opposite side of the polarizing prism 20, only a beam of polarized light whose plane of polarization coincides with the orientation of the polarizing prism 20 is allowed to pass through it. The light passing through the polarizing prism 20 enters the Faraday element 18 which rotates the plane of polarization 45°. In this case, the light is rotated in the same direction as the direction in which the light input in the forward direction is rotated. As a result, the light input in the opposite direction emerges from the optical isolator with its plane of polarization rotated 90°, i.e., orthogonal to that of the input light in the forward direction. When back-reflections are fed back to the LD operating at high speed, the operation of the LD may become unstable, leading to degradation of the light signal it produces. However, when the optical isolator shown in Figures 2A and 2B is inserted in the output light path of the LD, back-reflections do not affect the LD, and thus, unstable operation of the LD due to back-reflections can be prevented. However, in the prior art optical isolator shown in Figures 2A and 2B, since the Faraday element 18 positioned on the optical axis is enclosed in a hollow cylindrical permanent magnet, it has not been possible to reduce the thickness of the whole construction. Figures 3A to 3C show a construction according to one embodiment of the invention: Figure 3A is an external perspective view, Figure 3B is a cross-sectional view, and Figure 3C is a perspective view of a frame. The same parts as those shown in Figures 2A and 2B are designated by the same reference numerals, wherein 36 is a rod-like permanent magnet, 38 is a frame formed of a magnetic substance, such as soft iron, having high magnetic permeability, and preferably a weldable material, 40 is a light path hole formed in the frame 38, 42 is a recess formed in the frame 38, 44 are end portions, provided on both sides of the recess 42, for securing the permanent magnet, and 46 are protrusions formed in the recess 42. The Faraday element 18 and the polarizing prism 20 are the same as those shown in Figures 2A and 2B. As shown in Figures 3A and 3B, the permanent magnets 36 are respectively held between the end portions 44 at both ends of the two frames 38 and secured in place by adhesion or other means, while the polarizing prism 20 is inserted in the recess 42 in one frame and secured to it by adhesion or other means. The protrusions 46, which are, for example, of the same height, serve to guide the magnetic flux, emerging from the permanent magnets 36, toward the Faraday element 18 and also to hold the end of the polarizing prism 20 perpendicular to the optical axis 16. Further, the Faraday element 18 is secured to the polarizing prism 20. In this arrangement, the light path holes 40 formed in the frames at both ends, the Faraday element 18, and the polarizing prism 20 are disposed with their centers aligned with the same optical axis 16. When mounting this optical isolator in an LD package, etc., the optical isolator is secured parallel to the package surface by welding the frames to the package or by other means. In the optical isolator shown in Figures 3A to 3C, the magnetic flux lines emerging from the two rod-like permanent magnets 36 pass through one frame, then through the Faraday element 18 and polarizing prism 20, and return to the respective magnets through the other frame, thereby applying the required magnetic field to the Faraday element 18. Figure 4 illustrates the magnetic flux produced in the optical isolator of the present invention. The same parts as those shown in Figures 3A to 3C are designated by the same reference numerals. In Figure 4, the principal paths of the magnetic flux lines produced by the two permanent magnets 36 are illustrated in schematic form. Since the two permanent magnets 36 have the same direction of magnetization, the magnetic flux lines emerging from one end of each permanent magnet pass through the end portion 44 and protrusion 46 of the ferromagnetic frame at one end, then through the Faraday element 18 and polarizing prism 20 in the same direction, and return to the other end of the permanent magnet through the protrusion 46 and end portion 44 of the frame at the other end. At this time, the protrusions 46 serve to concentrate the magnetic flux on the Faraday element 18. In the optical isolator of the invention, rod-like permanent magnets are placed on both sides of the Faraday element. This arrangement allows a reduction in permanent magnet size, and hence the size of the optical isolator, permitting a low-profile construction. The optical isolator of the invention is provided with frames 38 made of a ferromagnetic substance. These frames help to concentrate the magnetic flux produced by the permanent magnets 36 on the Faraday element 18, ensuring stable and reliable polarization rotating operation in the Faraday rotator. This also permits a reduction in permanent magnet size, and hence the size of the optical isolator construction, for easy mounting. In the optical isolator of the invention, since one or more protrusions 46 are formed in the recess 42 of each frame 38, the magnetic flux produced by the permanent magnets 36 can be concentrated more effectively on the Faraday element 18, which also permits a reduction in permanent magnet size. In the embodiment shown in Figures 3A to 3C, two permanent magnets for applying a magnetic field to the Faraday rotator are arranged symmetrically to each other to provide uniform magnetic flux density throughout the Faraday element, but the invention is not limited to this arrangement. For example, three or more permanent magnets may be arranged so as to provide a uniform magnetic field. Further, the invention does not necessarily require the provision of two or more permanent magnets, but a single permanent magnet may be used if it can apply a sufficient magnetic field to the Faraday rotator. Figures 5A and 5B illustrate an example of an LD module incorporating the optical isolator of the present invention. Figure 5A is a top plan view, and Figure 5B is a side view. The numeral 50 is an LD package in which an LD as a light source and its driving circuitry are contained. The numeral 52 is an isolator section in which the optical isolator of the present invention is contained and through which, of the light emitted from the LD package, polarized light having a particular plane of polarization is passed for output. The numeral 54 is an optical fiber for transmitting the output light from the isolator section 32.

Claims: Faraday rotator comprising a Faraday element (18) with an optical axis (16), at least one permanent magnet (36) producing a magnetic flux, and at least one ferromagnetic member (38) for guiding a substantial proportion of the magnetic flux generated by said at least one permanent magnet (36) through said Faraday element (18), shaped such that the principle path of the magnetic flux lines through the Faraday element is parallel to and does not coincide with the optical axis of the Faraday element, characterized in that the at least one permanent magnet (36) is shaped rod-like, and that the at least one ferromagnetic member (38) is shaped such that the dimension of the ferromagnetic member corresponds to the dimension of the rod-like permanent magnet with regard to the direction perpendicular to the plane defined by the optical axis and the principle path of the magnetic flux lines. Faraday rotator according to claim 1 characterized in that the Faraday rotator comprises a single rod-like permanent magnet. Faraday rotator according to claim 1 characterized in that the Faraday rotator comprises two rod-like permanent magnets. Faraday rotator according to claim 3, characterized in that the permanent magnets are aligned in parallel to the optical axis having the same magnetization direction. Optical isolator comprising a Faraday rotator according to claim 1 - 4 and a polarizer (20). Optical isolator according to claim 5, characterized in that one ferromagnetic member is provided with a recess (42) for holding said polarizer. Optical isolator according to claim 5 or 6, characterized in that the Faraday element is with one side adjoint to the polarizer. Laser diode module comprising an optical isolator according to the claims 5 - 7 and a laser diode disposed near said Faraday element on the optical axis.

Inventors: ISHIZAKA TETSUO, KOMIYAMA MANABU, MASUKO TAKAYUKI, SATOH SHUNICHI, YOKOI SAEKO, ISHIZAKA, TETSUO, KOMIYAMA, MANABU, MASUKO, TAKAYUKI, SATOH, SHUNICHI, YOKOI, SAEKO, ISHIZAKA, TETSUO, C/O FUJITSU LIMITED, KOMIYAMA, MANABU, C/O FUJITSU LIMITED, MASUKO, TAKAYUKI, C/O FUJITSU LIMITED, SATOH, SHUNICHI, C/O FUJITSU LIMITED, YOKOI, SAEKO, C/O FUJITSU LIMITED
Applicants: FUJITSU LTD, FUJITSU LIMITED
Application year: 1994
Main classification: G02F1
IPC codes: G02F1, G02B27, G02B6